Nowadays, the liquid crystal display is used in a wide range of applications from television sets to video players, personal computers, word processing machines, projectors, to name a few, to exploit its high energy efficiency and low weight. The technology is however far from being mature, leaving even liquid crystal displays developed for practical use with problems that should be addressed.
The first problem is a low efficiency in the use of light. In a typical color liquid crystal display, a polarizer has a transmittance of ½ or lower, and a color filter has a transmittance of ⅓ or lower. Considering the aperture ratio and other factors, only 10% or less of the light emitted by the backlight is actually used; the ratio drops to 5% or even further under normal conditions. The low efficiency in light use leads directly to increases in power consumption. This is a problem that calls for serious attention, especially, if we consider the fact that we live in the 21 st century when environment- and energy-friendly technologies are increasingly essential.
To address this problem, several approaches have been made. One of them is to eliminate the backlight; these types of liquid crystal displays are termed reflective and includes no backlight power supply, effectively having cut down on power consumption. Nevertheless, the current reflective liquid crystal display can offer a contrast no higher than 20:1, which is hardly adequate to produce a really beautiful image.
Another approach is to eliminate the color filters which are a cause to reduce light transmission efficiency; although retaining the backlight, these types of liquid crystal displays boast improved light transmission efficiency, thanks to the elimination of the color filters. W. A. Crossland et al. suggest in SID 97 Digest, 837(1997), a way to implement the idea by replacing the color filters with a luminescent substance called photoluminescent. According to the method, however, the contrast is insufficiently low, and the use of ultraviolet light as the light source will likely damage the liquid crystal material and alignment film, which are issues that must be further addressed.
A field sequential color scheme is also publicly known which makes use of a backlight that switches colors between red, green, and blue in a time sequence (T. Uchida et al., Proc. IDRC, 37(1997)). This scheme again has problems, such as the requirement that liquid crystal with very high response be used.
Another problem with the liquid crystal display is that its image quality is inferior to that of the CRT. The liquid crystal display is a hold-type display and suffers persistent images and blurred edges when producing animation, whereas the CRT is an impulse-type display and is free from those shortcomings. The International Business Machines Corp.(IBM) has recently made a suggestion to solve these problems by the use of a liquid crystal display of an impulse type. The impulse-type liquid crystal display again has problems including those related with the response speed of the liquid crystal and the emission/extinguishment speed of light by the backlight.
To address the low efficiency in the use of light, the poor image quality, and other conventional problems, organic EL (Electro Luminescence) elements have been developed which shine in accordance with the pattern of liquid crystal display elements. A technology of this kind is disclosed in Japanese laid-open patent application 8-211832 (Tokukaihei 8-211832/1996, published on Aug. 20, 1996), for example.
The technology disclosed the laid-open patent application involves a liquid crystal display element section and an organic EL display element section. The liquid crystal display element section is chiefly made of liquid crystal and two sets of transparent electrodes positioned opposite each other to sandwich the liquid crystal. The organic EL display element section is chiefly made of an organic EL light emitting layer and two sets of transparent electrodes positioned opposite each other to sandwich the organic EL light emitting layer. The organic EL display element section is stacked on the liquid crystal display element section, and both display element sections are addressed by a single drive section to drive associated pixels of the liquid crystal display element section and the organic EL display element section. The liquid crystal display element section and the organic EL display element section thus display an identical image.
However, the technology requires both a matrix-type liquid crystal display element and a matrix-type organic EL display element stacked thereon, adding to the panel fabrication cost. An inevitably increased number of driver ICs adds further cost to the total production cost of the display apparatus.
Moreover, since a transparent substrate is positioned between the two display elements, the display apparatus provides poor visibility when viewed from an oblique angle. Attempts to improve the visibility will present other challenges: The aperture ratio must be reduced; and an ultra-thin, but expensive transparent substrate must be used.
The present invention, to solve these problems, has an objective to offer a novel optical control device capable of producing a visibility-improved impulse display without using a color filter and to offer a method of driving the device.